This invention relates to a process for the production of an ion exchange membrane and more particularly, it is concerned with an improved process for producing a heterogeneous ion exchange membrane having a high ion transport number in a high ion concentration.
An ion exchange membrane which comprises a particulate ion exchange resin attached to a support of synthetic fiber screen has been put to practical use. Such an ion exchange membrane supported with synthetic fiber is superior in mechanical strength to a homogeneous ion exchange membrane which consists of a membrane of ion exchange resin itself, but the mechanical strength of the ion exchange membrane is not sufficient, so that there are various limitations in its practical uses. For example, in order to improve the practicability of the ion exchange membrane by strengthening the chemical structure thereof it is necessary to increase the degree of cross-linking in the structure thereby to reduce the swellability of the membrane, which in turn reduces the concentration of the ion exchanging group, resulting in an increase of the specific resistance of the membrane, although the ion transport number is increased. Furthermore, the ion exchange membranes now on the market swell when they are dipped in aqueous solutions and shrink when they are dried, whereby the membranes become deformed and cracked and cannot be put to practical use. Accordingly, it is absolutely necessary to use the membranes in a wet state and at room temperature, which necessity is a bar to practical use thereof in an ion exchange apparatus.
Since such an ion exchange membrane generally lacks flexibility, there are some difficulties when using this ion exchange membrane in an ion exchange apparatus, and other possible uses of the ion exchange membrane in which the ion exchangeability is utilized are also precluded.
For the purpose of preparing a novel heterogeneous ion exchange membrane whereby the above described disadvantages can be overcome, it has hitherto been proposed to mix a finely powdered ion exchange material with a polyolefin resin as a matrix, mold the mixture and subject the molded article to post-treatment using hot water, as disclosed in Japanese Patent Publication No. 24262/1972 (corresponding to U.S. Pat. No. 3,627,703), Japanese Patent Public Disclosure No. 43888 (corresponding to U.S. Pat. No. 3,876,565) and Japanese Patent Application No. 53189/1974, published under Disclosure No. 145379/1975. The heterogeneous ion exchange membranes prepared by these processes can practically be used with some advantages as an ion exchange membrane for the desalting treatment of an aqueous solution having a relatively low ion concentration, but are not so satisfactory, for use as an ion exchange membrane for the desalting treatment of an aqueous solution having a relatively high concentration. That is to say, the heterogeneous ion exchange membrane prepared by the above described process shows an adequately low specific resistance, but, at the same time, has a disadvantage that the ion transport number is greatly lowered.